1. Field of the Invention
The present invention relates to a semi-transmissive liquid crystal display device and a portable terminal having a viewing angle that is switchable between a narrow viewing field mode and a wide viewing field mode.
2. Description of the Related Art
Liquid crystal display devices are widely used in direct-view monitors, projectors, and the like. In the currently used liquid crystal display devices, liquid crystals are sealed between two substrates, and the orientation of the liquid crystals is controlled by an electrical field applied to the liquid crystals, whereby information is displayed.
The liquid crystal display devices include transmissive liquid crystal display devices, in which light from a backlight is transmitted through a liquid crystal layer; reflective liquid crystal display devices that reflect outside light incident on the liquid crystal layer; and semi-transmissive liquid crystal display devices which have features of both the transmissive type and the reflective type, so as to transmit light from a backlight and to reflect incident light from the outside.
In particular, semi-transmissive liquid crystal display devices, which have both the good image quality of the transmissive type and the good ambient light visibility of the reflective type, currently constitute the mainstream in mobile device applications such as mobile phones and PDAs (Personal Digital Assistance). The semi-transmissive liquid crystal display devices can be further classified as being of internal semi-transmissive type in which light is reflected in the interior of the liquid crystal cells, or of external semi-transmissive type in which light is reflected at the exterior of the liquid crystal cells.
As one example of the internal semi-transmissive type, the liquid crystal display device disclosed in Japanese Laid-Open Patent Application 11-242226 has been proposed. FIG. 1 is a cross-sectional view schematically showing the cross sectional arrangement of a prior-art internal semi-transmissive liquid crystal element, and is based on FIG. 1 appearing in Japanese Laid-Open Patent Application 11-242226. As shown in FIG. 1, two substrates 102 are disposed facing each other over a backlight 109, with a polarizing plate 101 being provided to each of the substrates 102 on the side opposite from these opposing faces. The upper face of the first substrate disposed towards the backlight 109 comprises a reflective portion 121 provided with a concavo-convex reflecting electrode (internal reflecting electrode) 120, and a transmissive portion 122 provided with an electrode 103; the side of the second substrate that faces the first substrate is provided with an electrode 103 that extends through the reflective portion 121 and the transmissive portion 122; and a liquid crystal layer 104 is sealed between the two substrates 102. Specifically, in an internally reflective liquid crystal display element of such a design, there are provided within a single pixel a reflective portion 121 that has a concavo-convex reflecting electrode (internal reflecting electrode) 120 for reflecting incident light from the outside, and a transmissive portion 122 for transmitting the light of the backlight 109, so that both reflected light and transmitted light can be utilized for display. Since the appropriate thickness for the liquid crystals differs between the reflective portion and the transmissive portion, in most cases, liquid crystal thickness is made to differ in the respective portions. In FIG. 1, an insulating film 127 is disposed on the substrate in the reflective portion so that the gap between the concavo-convex reflecting electrode (internal reflecting electrode) 120 and the facing electrode 103 in the reflective portion 121 is smaller than the gap between the electrodes 103 in the transmissive portion 122, and the concavo-convex reflecting electrode (internal reflecting electrode) 120 is formed over the insulating film 127.
One example of the external semi-transmissive type is the liquid crystal display device disclosed in Japanese Laid-Open Patent Application 2000-180819. FIG. 2 is a cross-sectional view schematically showing the cross sectional arrangement of a prior-art external semi-transmissive liquid crystal element, and is based on FIG. 1 appearing in Japanese Laid-Open Patent Application 2000-180819. As shown in FIG. 2, the liquid crystal display device is provided with a backlight 109; over the backlight 109 two substrates 102 are arranged facing each other, electrodes 103 are provided on the opposing faces of the pair of substrates 102, and a liquid crystal layer 104 is sandwiched between the electrodes 103. A reflective polarizing plate 123 is disposed on the substrate 102 on the side that faces the backlight 109; and a polarizing plate 101 is provided so as to cover the surface of the reflective polarizing plate 123. A polarizing plate 101 is also provided to the substrate 102 that faces the first substrate. The plate is formed on the substrate side that is on the opposite side from the backlight 109. In this prior art example, transmissive display is carried out with the light of the backlight 109. During reflective display, however, specific polarized light of the light incident from the display face is reflected by the reflective polarizing plate 123, and the reflected light exits towards the observer, forming the displayed image. In this case, the transmissive portion and the reflective portion are in the same location, and a single pixel functions as a transmitting/reflective portion 124. A characteristic of systems in which reflective polarizing plates are used in the prior art is that the voltage-transmissivity curve (reflectivity) is reversed between transmissive display and reflective display.
In regard to the display element using a reflective polarizing plate disclosed in Japanese Laid-Open Patent Application 2000-193962, a description is given of a display element in which the same voltage-transmissivity (reflectivity) curves are obtained for a transmissive display and a reflective display through the use of a phase difference plate.
The liquid crystal display device disclosed in Japanese Laid-Open Patent Application 2003-098325 employs a different method based on an external reflective system. FIG. 3 is a cross-sectional view schematically showing the cross sectional arrangement of a prior-art external semi-transmissive liquid crystal element, and is based on FIG. 15 appearing in Japanese Laid-Open Patent Application 2003-098325. As shown in FIG. 3, above a backlight 109, two substrates 102 are arranged facing each other, electrodes 103 are provided to the substrates on the opposing faces thereof, and a liquid crystal layer 104 is sealed between the electrodes 103. Polarizing plates 101 are provided to each of the substrates 102 on the side opposite from their opposing faces. The polarizing plate 101 that faces the backlight 109 is provided with a semi-transmissive reflecting plate 125, which is disposed on the side of the plate that faces the backlight. The liquid crystal display element constitutes a transmitting/reflective portion 126. Specifically, in this prior-art liquid crystal display element, a semi-transmissive reflecting plate 125, rather than a polarized light reflecting plate, is disposed between the backlight 109 and the polarizing plate 101 disposed towards the backlight 109. In this case, in contrast to Japanese Laid-Open Patent Application 2000-180819, the same voltage-transmissivity (reflectivity) curves are obtained for a transmissive display and a reflective display.
In recent years, there has been a need for display devices to have a privacy protecting function whereby people other than the person viewing the device, namely, people close by, cannot view the device. For example, in the case of a banking terminal known as ATM (Automated Teller Machine) or the like, it is necessary to touch number buttons on a display device in order to input a Personal Identification Number, and such a display device must be prevented from being observed by others. Similarly, in the case of a mobile phone as well, there is a need for a function to prevent persons disposed close to the user from being able to see displayed information. Furthermore, in the case of PDAs and notebook personal computers (hereinafter also referred to as notebook PCs) as well, there is a similar need for a function to prevent nearby persons from being able to view the screen in trains or other forms of public transportation.
On the other hand, there are instances in which there is a need for a display device to be viewed by several individuals. For example, when television images are displayed on the screen of a mobile phone or the like, there are instances in which it would be desirable to show the mobile phone to a nearby individual in addition to the owner. There are also instances in which a data screen of a notebook PC is viewed by several people.
Consequently, a display device may have a narrow viewing field mode for use in individual viewing of highly confidential information, and a wide viewing field mode for use in viewing highly public information by several people. Moreover, in the case of mobile phones, PDAs, and notebook PCs, there is a need for a display device switchable between these display modes.
The liquid crystal display device disclosed in Japanese Laid-Open Patent Application 10-153968 is a display device capable of being switched between a narrow viewing field mode and a wide viewing field mode. FIG. 4 is a plan view showing the pixel arrangement of the liquid crystal display device disclosed in Japanese Laid-Open Patent Application 10-153968. FIG. 5 is the voltage-transmissivity plot for a wide viewing field area during viewing in a narrow field as described in the patent document, and FIG. 6 is the voltage-transmissivity plot for a wide viewing field area during viewing in a wide field as described in the patent document.
As shown in FIG. 4, the active matrix liquid crystal display device disclosed in Japanese Laid-Open Patent Application 10-153968 comprises a plurality of pixels 111 in which liquid crystals are sealed between transparent electrodes and are arranged in matrix form. Each pixel 111 comprises a first pixel region 112 connected to a control line 116, and a second pixel region 113 connected to the first pixel region via a capacitor 114; and a switching element 115 is provided between the first pixel region 112 and the second pixel region 113.
Here, operation when the liquid crystal mode is the TN (Twisted Nematic) mode shall be described. During viewing in a narrow field, the switching element 115 is shorted. Since the first pixel region 112 and the second pixel region 113 are directly connected, the same voltage (V1) as that of the control line 116 is fed to the first pixel region 112 and the second pixel region 113. Since the first pixel electrode in the first pixel region 112 and the second pixel electrode in the second pixel region 113 are at identical voltage, operation is the same as in normal TN mode. As shown in FIG. 5, the grayscale inversion characteristic of the TN mode appears in the voltage-transmissivity curve in the wide viewing range of the pixel 111 as a whole. This characteristic combines the voltage-transmissivity curves of the first pixel region 112 and the second pixel region 113.
On other hand, during viewing in a wide field, the switching element 115 is open. Since the first pixel region 112 and the second pixel region 113 are connected via the capacitor 114, the driving voltage applied to the control line 116 (voltage V1) is supplied unchanged to the first pixel region 112 while being supplied to the second pixel region 113 via the capacitor 114 (voltage V2). Thus, the voltage supplied to the first pixel region 112 is different from the voltage supplied to the second pixel region 113. Specifically, the second pixel voltage in the second pixel region 113 connected via the capacitor 114 has an absolute value that is less than the value of the first pixel voltage in the first pixel region 112 (|V1|>|V2|). As shown in FIG. 6, by means of applying different voltages to the first pixel region 112 and the second pixel region 113, the voltage-transmissivity curve 118 in the wide viewing range of first pixel region 112 has a waveform different from that of the voltage-transmissivity curve 119 in the wide viewing range of second pixel region 113; and the voltage-transmissivity curve 117b in the wide viewing range of the pixel 111 as a whole, which curve combines the voltage-transmissivity curve of the first pixel region 112 and the voltage-transmissivity curve of the second pixel region 113, assumes a smooth curve free from grayscale inversion.
That is, in the prior art disclosed in Japanese Laid-Open Patent Application 10-153968, the viewing angle is switched between a wide viewing field mode and a narrow viewing field mode by changing the voltage-transmissivity characteristics of the pixels and varying the viewing angle characteristics of the liquid crystal panel. Table 1 summarizes differences in the voltages applied to the first pixel region and the second pixel region in the wide viewing field mode in relation to the narrow viewing field mode. In the wide viewing field mode, the voltages applied to the first pixel region and the second pixel region are different, while in the narrow viewing field mode the voltages are the same.
TABLE 1Wide viewing field modeNarrow viewing field modeFirst pixelSecond pixelFirst pixelSecond pixelregionregionregionregionAppliedAppliedAppliedAppliedvoltage V1voltage V2voltage V1voltage V1
In Japanese Laid-Open Patent Application 09-006289, there is described a liquid crystal display device wherein single pixels are composed of a plurality of sub-pixels having different orientation characteristics, with the sub-pixels being connected to control lines for independently controlling the operation thereof. The operator operates a switch whereby sub-pixels for applying control signals via a control line group are selected from the pixels, and the viewing angle of the display element is switched between a wide viewing angle designed to facilitate viewing by the operator, and a narrow viewing angle designed to prevent observation by a third party.
However, the display devices that enable switching between a narrow viewing field mode and a wide viewing field mode have problems such as the following. First, in the liquid crystal display device disclosed in Japanese Laid-Open Patent Application 10-153968, in the narrow viewing field mode of a transmissive display, grayscale inversion is produced for an observer whose views the display at a wide viewing angle. However, depending on the type of display, the observer would still be able to view the display at a wide viewing angle despite the presence of grayscale inversion. Furthermore, switching of the viewing angle in a reflective display is not disclosed in relation to this system, and no measures are envisioned that would allow the viewing angle of the semi-transmissive liquid crystal display device to be switched. In the liquid crystal display device disclosed in Japanese Laid-Open Patent Application 09-006289 as well, there is no mention of switching the viewing angles of a reflective display, and no measures are envisioned that would allow the viewing angle of the semi-transmissive liquid crystal display device to be switched.